What IS good, and how do we determine it?

[Randy Carson]

The belief is that they never actually had sex.

and that somehow makes a 30yo man marrying a 12yo ok?

Sure it was smart for Mary and Joseph to Mary.

The Protoevangelium of James was written around A.D. 120, when some of those who had known the apostles were still alive. It records that Mary was dedicated before her birth to serve the Lord in the temple as Samuel had been dedicated by his mother (1 Sam. 1:11). This required perpetual virginity of Mary so that she could completely devote herself to the service of the Lord.

According to the Protoevangelium, concerns about ceremonial cleanliness required that Mary have a male protector who would respect her vow of virginity. Joseph was "chosen by lot to take into [his] keeping the Virgin of the Lord." His duty to guard Mary was taken so seriously that when Mary conceived, Joseph had to answer to the temple authorities. So Mary’s betrothal to Joseph was not in conflict with her vow of virginity.

Mary was ever-virgin. No sex with Joseph.

Hope this helps.